Transmission line structures in microwave circuits are often a large part of the overall circuit size. Since the cost of a microwave circuit generally increases as its size increases, minimizing the size of transmission line structures can be of significant importance for many applications of microwave circuits.
Physical size of a transmission line is usually governed by its desired electrical characteristics, and in many cases—by a target electrical length of the transmission line. The electrical length of a transmission line is proportional to a ration of its physical length to a wavelength of the guided electromagnetic mode propagating along the transmission line. For many applications, such as impedance matching or in a coupler, transmission lines of specific electrical lengths are required, limiting thus a minimum achievable circuit size for a type of transmission line used in a particular application. This size limitation can be overcome using a transmission line structure that is physically shorter and loading it with reactive loading to achieve an electrical length equivalent to a longer, unloaded transmission line.
Different lengths of transmission lines have different total inductances and total capacitances, and therefore perform differently even at the same frequency. The size-reduced transmission line structures can be made electrically equivalent to standard transmission lines by compensating for the lower total inductance and capacitance of a shortened transmission line relative to a longer transmission line. Hettak et al, in an article entitled “The use of uniplanar technology to reduced microwave circuit size”, Microwave Journal, May 2001 which is included herein by reference, has shown that, whereas capacitively loading the ends of a shortened transmission line compensates for its lower total capacitance, the shortened transmission line has to have a higher characteristic impedance to compensate for its lower total inductance. This compensation results in a size-reduced structure having, at a pre-determined operating frequency, the same effective characteristic impedance and effective electrical length as a longer transmission line.
These size-reduced transmission line structures result in smaller circuits maintaining a target electrical performance within a given frequency range.
U.S. Pat. No. 4,127,832 issued to Riblet discloses a directional coupler preferably constructed in stripline or microstrip media comprising four sections of transmission line interconnected so as to form at their junctions four ports of the coupler, having four capacitive elements such as stripline or microstrip stubs connected at each junction so that physical length of the four sections of transmission line is reduced. In a similar approach, Sakagami et al, in an article entitled “Reduced branch-line coupler using eight two-step stubs”, IEE Proc.-Microw. Antennas Propag., Vol. 146, No. 6, December 1999, disclosed a shortened microstrip transmission line with capacitive loading using shunt microstrip stubs.
Hirota et al, in an article entitled “Reduced-size branch-line and rat-race hybrids for uniplanar MMIC's”, IEEE Transactions On Microwave Theory And Techniques, Vol. 38, No. 3, March 1990, disclosed a shortened coplanar waveguide (CPW) transmission line with capacitive loading using shunt Metal-Insulator-Metal (MIM) capacitors.
Hettak et al, 2001, disclosed a shortened uniplanar transmission line with capacitive loading using shunt uniplanar stubs.
The aforementioned approaches to transmission line size reduction have their advantages and disadvantages.
MIM capacitors at high frequencies, for example, in microwave and millimeter-wave wavelength regions, can be difficult to model and may be susceptible to fabrication process deviations. In these instances, the electrical performance of a size-reduced transmission line may be negatively affected.
Standard microstrip stubs suffer from at least two negative aspects that limit a total amount of size reduction. Firstly, for a given amount of capacitive loading, a physical length of the stub providing the loading may offset the size reduction of the loaded line. Secondly, standard microstrip stubs must be placed far enough apart to prevent electromagnetic coupling between them, usually at least a substrate thickness apart. This minimum spacing also limits the total amount of size-reduction.
Using uniplanar stubs partially overcomes the limitations of standard microstrip stubs. Uniplanar stubs couple less to each other due to a uniplanar ground conductor that separates them. Uniplanar stubs can also have lower characteristic impedance compared to standard microstrip stubs. Hence, uniplanar transmission lines and stubs allows more significant size-reduction compared to standard microstrip media wherein signal and ground conductors are disposed on opposite sides of a relatively thick substrate. Nonetheless, size-reduction using uniplanar stubs is still limited by their minimum realizable characteristic impedance and a minimum spacing between them required for electromagnetic isolation.
Recently, microwave circuits combining uniplanar transmission lines and thin-film microstrip (TFMS) stubs were disclosed wherein the microstrip stubs have signal conductors disposed in a different layer than the uniplanar transmission lines. T. Le Nadan et al, in an article entitled “Optimization and miniaturization of filter/antenna multi-function module using a composite ceramic/foam substrate”, 1999 IEEE International Microwave Symposium, disclosed using half-wavelength TFMS stub resonators connected to a uniplanar transmission line to form a band-pass filter connect to a patch antenna. TFMS stubs were used in Le Nadan solely to increase the isolation between the filter and the antenna.